During the production of polymers in general and polyesters in particular, as is known, thermal and oxidative degradative reactions also take place at the same time, through which chromophoric groups are formed in the polymer molecules. These lead to a yellowish discoloration of the polymer (see e.g., “Polyesterfasern, Chemie und Technologie,” Dr. Hermann Ludewig, Akademie-Verlag, Berlin, 1975; “Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters,” John Scheiers and Timothy E. Long (Ed.), John Wiley & Sons, 2003). This slight yellow coloration can also be detected well metrologically. If no dye is added to the polyester during production, the b color value then measured (Hunter Method) is between 2-4 units.
There are essentially two possibilities for minimizing degradative reactions of this type, namely working under a protective gas atmosphere and lowering the reaction temperature. While the former can still be realized with reasonable expenditure, the latter is possible only to a limited extent. Since the temperature is an essential factor for the reaction velocity, it has to be carefully considered how far a temperature reduction or the color improvement achieved thereby is still associated with a technically and economically reasonable reaction velocity. There has therefore been no lack of attempts to prevent a color error in the product. One established method for this polymer color technology requires the use of blue dyes. Through the addition of a suitable quantity of blue dye, the yellow base tone of the polymer is shifted to a visual impression neutral in color. The neutral color shade is particularly important for packaging materials, in particular for foodstuffs and beverages.
Cobalt compounds often used in dye technology, in particular cobalt acetate, are known as blue colorants, which as a rule are used jointly with organic dyes. However blue, red and violet organic dyes are also used alone.
For example, the use of cobalt compounds is disclosed in EP 0 061 414, where a production method for a polyester resin is described, which contains a cobalt compound, a phosphorous compound and an antimony compound, and is used for producing articles with high clarity and a neutral color tone. The cobalt compound is the colorant component therein, the phosphorous compound is used to suppress the catalytic effect of the cobalt compound, and the antimony compound is used as a catalyst for the polycondensation.
U.S. Pat. No. 6,793,083 describes the production of a clear polyester neutral in color using a preblend of cobalt and phosphorous which is added to the production process either at the end of the esterification stage or at the start of the polycondensation stage. Here, too, cobalt is the colorant component and phosphorous is the stabilizer for cobalt.
A general description of the use of organic dyes for adjusting the color neutrality in polyesters is found, for example, in “Polyester Bottle Resins: Production, Processing, Properties and Recycling,” Dr. Ulrich K. Thiele, PETplanet Publisher GmbH, Heidelberg, 2007.
EP 0 854 160 discloses a diethylene glycol modified PET copolymer of improved clarity with which cobalt compounds, mixtures of blue and red anthraquinone derivatives and isoquinoline derivatives are used as colorants for color adjustment.
U.S. Pat. No. 5,372,864 discloses different blue and red anthraquinone derivatives and anthrapyridone derivatives which can be used as dye mixtures in order to adjust a neutral or slightly bluish color in polyesters. The thermally stable dyes can thereby also carry reactive groups that make it possible to bond the dyes to the polyester during the polycondensation. The yellowing is thereby corrected via the quantity of color mixture used, a coloring in the red-green direction can be equalized by means of the composition of the color mixture. However, there is no separate consideration here of the b value of the polymer in the L, a, b color space (measured according to Hunter), which is a gauge of the yellowing, and of the a value, which characterizes a red-green shift. It is not indicated either how the a and b values are to be influenced in a targeted manner separately from one another, or it is not even possible due to the concentrations provided. The determination of dye composition and quantity for the most exact possible neutralization of a color tone in the polymer accordingly always requires considerable testing expenditure, since the a value achieved changes with the b value or the dye mixtures and quantities used for that purpose. According to the Hunter L,a,b color scale, “L” has a value of 0 to 100 and is a measure of reflecting light diffusion, the values “a” and “b” have no numerical limits, positive “a” is red, negative “a” is green, positive “b” is yellow, and negative “b” is blue.
The known cobalt compounds as colorants, of which in particular cobalt acetate is used for foodstuff packaging, have a number of disadvantages. In addition to its property as a blue-violet colorant, cobalt also has catalytic properties and is used among other things as a esterification catalyst. However, this property is undesirable in the use of cobalt as a colorant. The cobalt compound is therefore stabilized with the addition of a stabilizer, e.g., phosphorous from phosphoric acid, and the catalytic reaction of the cobalt is thus suppressed as far as possible. However, at the same time the formation and precipitation of cobalt phosphates is possible, which in turn causes an increase in the turbidity values in the polymer. Particularly for packaging materials, in particular beverage bottles, a turbidity of this type is naturally unacceptable. Furthermore, an elimination of acetic acid from the cobalt acetate often uses can lead to attacks on the reactors, pipelines and the distillation.
The food regulations, which completely exclude certain compounds or at least limit the concentration thereof, are another problem. This problem applies not only to cobalt compounds but also to organic dyes. Only recently have colorants that have international food approvals been increasingly developed and introduced to the market. Until now a freely selectable color design without concessions to the colors L, a and b (according to Hunter) and the turbidity values in the polymer has therefore hardly been possible.
Today increasing importance is also attached to the lowest possible metal content in polymers for foodstuff packaging. Although the food regulations do not stipulate the total metal content, attempts are made to circumvent problems with the migration of metals into the foodstuff. It is therefore advantageous if the cobalt compounds can be replaced by organic dyes. However, in the past only the b value could be adjusted very exactly with the use of blue/red dye combinations. The development of the a value and the L value then resulted from the dye combination. An independent adjustment of the a and b value was not possible.